[GStreamer][MSE] Dumps pipeline for each buffer pushed on msesrc pads

https://bugs.webkit.org/show_bug.cgi?id=272814

Reviewed by Alicia Boya Garcia.

Dump the pipeline only for the first buffer pushed on each pad. Doing that for every buffer has a
high CPU cost and doesn't seem very useful :)

* Source/WebCore/platform/graphics/gstreamer/mse/WebKitMediaSourceGStreamer.cpp:
(webKitMediaSrcLoop):

Canonical link: https://commits.webkit.org/277638@main

Source/WebCore/platform/graphics/gstreamer/mse/WebKitMediaSourceGStreamer.cpp

@@ -587,7 +587,7 @@ static void webKitMediaSrcLoop(void* userData)
             GST_ERROR_OBJECT(pad, "Pushing buffer returned %s", gst_flow_get_name(result));
             dumpPipeline("pushing-buffer-failed", stream);
             gst_pad_pause_task(pad);
-        } else {
+        } else if (pushingFirstBuffer) {
             GST_DEBUG_OBJECT(pad, "First buffer on this pad was pushed (ret = %s).", gst_flow_get_name(result));
             dumpPipeline("first-frame-after", stream);
         }